The present invention relates to a wood-type golf club head having an improved flight direction performance for hit ball.
In order to improve the flight direction performance of a wood-type golf club head, it is proposed to increase the moment of inertia (to be exact, the moment of inertia of a club head about the vertical axis passing through the center of gravity of the club head). That is to say, a gear effect produced by off-center hit that a golf ball has hit a club head outside a sweet spot of the club head, for example, near the toe or heel of the club head, is suppressed by increasing the moment of inertia, whereby the side spin amount of the struck golf ball is decreased to stabilize the direction performance.
The gear effect produced when hitting a golf ball by a right-handed golfer is briefly explained below (all explanations made herein being for right-handed golfers). For example, if a golf ball “b” is struck by a club head “a” at a position on the toe side of the club face as shown in FIG. 10(A), the club head “a” rotates clockwise about the club head's center of gravity by a force receiving from the ball. Since the ball “b” and the club face are in contact with each other at that time, a side spin which causes the ball to rotate in the counterclockwise direction which is opposite to the rotation of the club head “a” (so-called hook spin) is imparted to the ball “b” by a frictional force between the ball and the face. Therefore, the ball tends to curve toward the left of the intended line of flight. Such an action is called “gear effect” with likening the head “a” and the ball “b” to a pair of engaged gears. When the club head strikes the ball “b” on a heel hit as shown in FIG. 10(B), a gear effect of the reverse rotation to the hook spin is produced to impart a side spin of the clockwise rotation (so-called slice spin) to the ball “b”. The slice spin tends to cause the ball to curve toward the right of the intended line of flight.
In order to improve the directionality for the hit ball, it is also proposed to make the depth of the center of gravity small, as disclosed in JP 9-140836 A and U.S. Pat. No. 6,913,546 B2. The depth of the center of gravity is a horizontal length between the center of gravity of the head and the sweet spot on the face of the head in the front-rear direction of the head. The moment rotating the club head around the center of gravity on an off-center hit increases as the depth of the center of gravity increases. Therefore, if the depth of the center of gravity is large, the gear effect becomes large and the side spin amount of the hit ball tends to increase. In improving the directionality of hit ball by suppressing the gear effect, it is desirable to decrease the depth of the center of gravity.
In JP 9-140836 A, it is proposed to decrease the depth of the center of gravity by thickening the face portion of the head. However, a thick face portion may deteriorate the repellency of the face portion to reduce the flight distance of ball. Further, since the proposed club head has a volume of 220 to 320 cm3, it is inferred that the moment of inertia of the head is small and, of course, no consideration is given to a relationship between the depth of the center of gravity and the moment of inertia.
In U.S. Pat. No. 6,913,546 B2, it is proposed to decrease the depth of the center of gravity by using a metallic material having a high specific gravity as a material for preparing the face portion of the head. However, the use of a metallic material having a high specific gravity has a limit in increasing the head volume and, for example, it is difficult to produce club heads having a volume of 420 cm3 or more. Further, since the face portion is heavy, it is required for increasing the moment of inertia to dispose a heavier material at a peripheral portion of the head, whereby the head weight becomes too large, so the head speed during the swing is lowered and it becomes difficult to perform the swing to impair the directional stability.
It is an object of the present invention to provide a wood-type golf club head having an improved directional stability without lowering the flight distance of hit ball.
This and other objects of the present invention will become apparent from the description hereinafter.